Exercise-Crazed Mice Are Poor Thinkers

Exercise may improve our brains as well as our bodies, but it's possible to get too much of a good thing.

Those characters who become so addicted to exercise that they don't seem to know how to stop may actually be impairing their brain's ability to learn new tasks.

That's no excuse to stay on the couch, according to the lead author of a series of new studies showing that exercise increases the chemicals in the brain that help brain cells communicate with each other. It also helps the brain grow new neurons in the region known as the hippocampus, which controls learning and memory.

So Justin Rhodes, a neuro-scientist at the Oregon Health & Science University in Portland was a bit surprised when his data showed that mice that had become addicted to exercise had a terrible time finding their way through a maze. Other mice that exercised at a "normal" level breezed through the test with no problems. The researchers report their findings in two current professional journals, Neuroscience, and Behavioral Neuroscience.

Born to Run

Since the hyperactive mice were creating new neurons and producing more of the "good chemical" that the brain needs to function properly, called "BDNF (brain derived neurotrophic factor) it seems logical that they would also learn how to get through the maze better than the couch potatoes. But they didn't.

The finding raises as many questions as answers, and there's no real evidence yet that humans experience the same mental impairment from addictive exercise as mice, but Rhodes believes that's probably the case.

"All we know is mice," he says. But, he adds, earlier research at the Salk Institute in San Diego found that humans also produce more BDNF and generate new neurons when they exercise, just like mice. So it's not much of a leap to conclude that too much exercise might also impair mental abilities in humans, just as it does in mice.

But it's worth noting that the mice that Rhodes has been studying for years now are real nut cases. They are so addicted to exercise that they run three times as much on a running wheel as normal mice.

"It's sort of a pathology," Rhodes says. "They are abnormal animals. They are addicted to exercise."

Rhodes began his research while working on his doctorate at the University of Wisconsin. His colleagues there had selectively bred mice to propagate a strain that really wanted to get on that wheel and run.

"It's just like breeding horses to run fast or breeding dogs to look cute," Rhodes says. "We only bred the ones that ran the most revolutions."

They ended up with 30 generations of exercise freaks, each generation more addicted than the last. Some of these superachievers ran for more than 20 kilometers every night over a seven-day period.

At the end of the experiment, when the mice were denied access to the running wheel, experiments revealed an enormous level of activity in the same part of the brain that becomes active when animals are craving for drugs.

"You see the same sort of brainwaves lighting up," Rhodes says.

Mysterious Agent

Rhodes headed off to the Salk Institute, with a suitcase full of mouse brains, to measure the level of BDNF and to see if there was any evidence of new neurons. Bingo on both counts. In fact, BDNF increased by as much as 171 percent in some of the mice.

That, of course, raises the question of why they don't perform better mentally. No one knows for sure at this point, because no one knows the "functional significance of the new neurons and BDNF," Rhodes says.

In other words, no one knows exactly what that stuff is supposed to do.

But it seems likely that both are facilitators for learning, and part of the experiment supports that. In addition to the hyperactive mice, the researchers also had a bunch of "normal" mice that spent a reasonable amount of time on the running wheel, because all rodents like to run on a wheel, but they weren't addicted to it.

The normal mice also increased their good chemicals and grew new neurons while exercising, and they also improved their ability to learn how to get through a maze.

So a normal level of exercise does help in learning, at least for a mouse.

Exercise in Moderation

What does it all add up to?

"What our study basically confirms is normal exercise is good for the brain and enhances learning," Rhodes says. "But when you take an animal that is running three times as much as normal, an exercise addict, that actually has a negative effect."

Of course, it's possible that the reason the hyperactive mice performed so poorly in the mental test has nothing to do with brain chemicals or new neurons. Maybe, Rhodes speculates, they just couldn't think about anything but running.

Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.